In the transmission of signals via optical fibers, both analog and digital modulation of the light source is utilized in different applications. Recently, digital transmission has become more prominent due to its inherent error reducing capabilities. Considering the latter, the digital information to be transmitted modulates a light source, such as a light emitting diode (LED) or a solid-state laser diode (ILD). The light from such a source is propagated through the optical fiber or light pipe by total internal reflection. At the receiving terminus, the light is directed upon a photodetector. The latter may be, for example, either a PIN photodiode or an avalanche photodiode (APD). The small energy levels produced by the photodetector are thus amplified and converted back to digital form for further use.
While the transmitters for use in such digital systems are easily designed and relatively low cost, the receivers involve complicated circuits and are expensive. The reason for this stems from the relatively small amount of light arriving at the receiving terminus and the limited sensitivity of the photodetectors. The former results from two major causes, namely attenuation within the light pipe itself and input coupling losses where only a fraction of the source's radiant power is actually coupled into the fiber and waveguided. Accordingly, it is apparent that a large amount of amplification is needed to bring the small signal input from the photodetector to a useful level.
Several problems arise in the design of amplifiers for digital systems using photodetectors as their input sources. One of these involves a noise component within PIN photodiodes caused by fluctuations in dark current. The latter current flows through the diode-biasing circuit when no light is incident on the photodiode. An average dc value for dark current is usually specified by the manufacturer at a given temperature and bias voltage. However, it is known that dark current shot-noise power varies linearly with this average. Dark current increases with temperature and substantially doubles in amplitude for every 10 degrees Celsius increase in operating temperature. Another problem in amplifier design stems from the initial dc offset inherent in all operational amplifiers. When the aforementioned variables are in their worst-case direction, it is impossible to predict over a long period of time, the integrity of the signal levels exiting the receiver.
In view of the foregoing, it is apparent that the need exists for a low-cost optical system for dc transmission of information. The receiver of the present invention, characterized by simplicity of design and economy, provides the required amplification of the photodetector signals, while rejecting any long-term drift. As such, it may be advantageously employed in the aforementioned system.